devtools: Show clients where they can set breakpoints (#37667)

devtools clients query source actors to determine where the user can set
breakpoints in a source. there are two relevant requests here:
`getBreakableLines` controls which line numbers can be clicked in the
margin, and once a line number is clicked,
`getBreakpointPositionsCompressed` controls where to show breakpoint
buttons within that line.

this patch handles those requests by querying the [SpiderMonkey Debugger
API](https://firefox-source-docs.mozilla.org/js/Debugger/) for that
information:
- devtools sends its script thread a GetPossibleBreakpoints message for
the source’s
[`spidermonkey_id`](https://firefox-source-docs.mozilla.org/js/Debugger/Debugger.Source.html#id)
- the script thread fires a `getPossibleBreakpoints` event into its
debugger global
- the debugger script looks up the
[root](https://firefox-source-docs.mozilla.org/js/Debugger/Debugger.html#onnewscript-script-global)
[Debugger.Script](https://firefox-source-docs.mozilla.org/js/Debugger/Debugger.Script.html#getpossiblebreakpoints-query)
for that source, calls
[getPossibleBreakpoints()](https://firefox-source-docs.mozilla.org/js/Debugger/Debugger.Script.html#getpossiblebreakpoints-query),
and returns the result via
DebuggerGlobalScope#getPossibleBreakpointsResult()
- that method takes the pending result sender, and sends the result back
to devtools
- devtools massages the result into the format required by the request,
and replies to the client

as a result, users of the Firefox devtools client can now set
breakpoints, though they don’t have any effect.

Testing: this patch adds new devtools tests
Fixes: part of #36027

<img width="1433" height="1328" alt="image"
src="https://github.com/user-attachments/assets/f0cd31e0-742f-44d3-8c5d-ceedd9a2706d"
/>

---------

Signed-off-by: Delan Azabani <dazabani@igalia.com>
Co-authored-by: atbrakhi <atbrakhi@igalia.com>

resources/debugger.js

@@ -5,6 +5,7 @@ if ("dbg" in this) {
 const dbg = new Debugger;
 const debuggeesToPipelineIds = new Map;
 const debuggeesToWorkerIds = new Map;
+const sourceIdsToScripts = new Map;
 
 dbg.uncaughtExceptionHook = function(error) {
     console.error(`[debugger] Uncaught exception at ${error.fileName}:${error.lineNumber}:${error.columnNumber}: ${error.name}: ${error.message}`);
@@ -12,6 +13,7 @@ dbg.uncaughtExceptionHook = function(error) {
 
 dbg.onNewScript = function(script, /* undefined; seems to be `script.global` now */ global) {
     // TODO: handle wasm (`script.source.introductionType == wasm`)
+    sourceIdsToScripts.set(script.source.id, script);
     notifyNewSource({
         pipelineId: debuggeesToPipelineIds.get(script.global),
         workerId: debuggeesToWorkerIds.get(script.global),
@@ -33,3 +35,8 @@ addEventListener("addDebuggee", event => {
     });
     debuggeesToWorkerIds.set(debuggerObject, workerId);
 });
+
+addEventListener("getPossibleBreakpoints", event => {
+    const {spidermonkeyId} = event;
+    getPossibleBreakpointsResult(event, sourceIdsToScripts.get(spidermonkeyId).getPossibleBreakpoints(/* TODO: `query` */));
+});